I. Field of the Invention
This invention pertains to novel cementitious and non-cementitious ferrofluids having properties which make them useful in cementing wells. This invention is more specifically related to cementitious ferrofluids and a proces of using such fluids in cementing oil wells.
II. Technology Review
A variety of compositions have been described as stable ferrofluids and a wide variety of cementitious compositions have been used in known processes for cementing wells. However, the two fields of technology are essentially unrelated.
U.S. Pat. No. 4,356,098 (Chagnon) is a reference in the ferrofluid technology which describes certain stable ferrofluid compositions and a method of making same. Chagnon indicates that ferrofluids are ferromagnetic liquids which typically comprise a colloidal dispersion of finely-divided magnetic particles, such as iron, gamma-Fe.sub.2 O.sub.3 (maghemite), Fe.sub.3 O.sub.4 (magnetite) and combinations thereof, of subdomain size, such as, for example, 10 to 800 Angstroms, and more particularly 50 to 500 Angstroms, dispersed in a liquid through the use of a surfactant-type material. Chagnon states that typically ferrofluids are remarkably unaffected by the presence of applied magnetic fields or by other force fields in that the magnetic particles remain uniformly dispersed throughout the liquid carrier. Such ferrofluid compositions are widely known, and typical ferrofluid compositions are described, for example, in U.S. Pat. No. 3,764,540 and a process for making such materials is described in U.S. Pat. No. 3,917,538 and U.S. Pat. No. 4,019,994. Chagnon and the recited references therefore describe ferrofluids in which the only suspended particles are magnetic particles and the liquid medium is generally organic rather than aqueous.
The well cementing technology is replete with references to hydraulic cements and methods of using such cements in cementing wells. In this technology, cement slurries are used to fill the void space between the casing or pipe in the walls of the borehole penetrating a subterranean formation; a process called "well cementing" in the industry. In using such cement slurries, a line or string of pipe is inserted into the borehole and a cement slurry is pumped downwardly through the pipe into the bottom of the borehole and then upwardly along with outside of the casing or pipe displacing drilling mud from the annular space. The cement slurry is then displaced from the interior of the pipe before it hardens; this is normally accomplished by injecting a liquid medium behind the cement slurry and using it as a "hydraulic fluid" to force the remaining cement out of the pipe and into the annulus. In another but less common technique, the borehole is filled with the cement slurry and the pipe or casing (normally with the end sealed) is lowered into the hole. Cementing techniques are more fully described by D. K. Smith in "Cementing" , Monograph Vol. IV, Henry L. Doherty Series, Society of Petroleum Engineers of AIME, New York (1976).
Well cementing operations place very rigorous demands upon the cement slurry, and a special area of technology has been developed to supply cements capable of meeting such demands. The person of ordinary skill in the art relating to the present invention is, therefore, one skilled in the formulation and use of cement slurries in well cementing.
The skilled artisan will recognize that additives to conventional cements and concretes used in the construction industry may or may not have applicability in cement slurries for well cementing. The conditions of use are sufficiently different that use in one area does not necessarily suggest use in the other. For example, styrene-butadiene latex had been used in cements and concretes in the construction industry for many years, but when formulated in an oil well cement slurry with certain fluid-loss additives, the resulting cement slurry proved unusually effective in preventing gas channelling. See U.S. Pat. No. 4,537,918 (Parcevaux et al.).
Channelling of gases upwardly through the setting cement slurry is one problem, but a similar "channelling" can occur if the cement slurry does not form a good bond with the casing and with the formation wall. If drilling mud is not uniformly and completely displaced from the annulus, a "microannulus" will form when the slurry fails to bond properly. Various techniques have been used but the most common commercial techniques used to combat this problem involve chemical washes or spacers and/or expansive cements. See, for example, U.S. Pat. No. 4,207,194 (Sharpe et al.) which illustrates the use of chemical washes and spacers. Such chemical washes are injected as a preflush ahead of the cement slurry and are thereby used to displace the drilling mud and "wash" the walls of the formation and casing before being contacted by the cement slurry. See also U.S. Pat. No. 4,328,036 (Nelson et al.) which illustrates the use of expansive cements. Another method utilizes sonic or energy-carrying waves to displace the drilling mud from the walls of the casing and formation. See U.S. Pat. No. 4,093,028 (Brandon). These methods work to a greater or lesser degree, but a need still exists for a method of removing the drilling mud from the casing and formation walls so as to promote better bonding of the cement slurry thereto. The drilling muds and cement slurries are typically incompatible, particularly when the drilling mud is an oil-base drilling mud.